Complete dominance
Let T represent the gene for tallness while t represent the gene for dwarfness.
During gamete formation (gametogenesis) in the dwarf plant, each gene in the pair segregates into different gametes.
When the male and female gametes fuse during fertilisation, the offspring produced contains the same number of genes as in each parent.
The inheritance of dwarfness in the pea plant can be illustrated diagrammatically using a genetic cross.
Similarly, the pair of genes in the pure breed tall pea plants will segregate into different gametes during gametogenesis. Thus, when self-fertilized, the resulting seeds will have half the number of genes from each parent.
When the purebred tall plant is crossed with dwarf plants, the resulting seeds grow into tall plants only. These offspring represent the first generation known as the first filial generation (F1). The dwarf characteristic is not represented in the offspring.
From the genetic crosses above, the following components of a genetic cross are obtained;
- Parental phenotypes.
- Parental genotypes
- Crossing (X) should be shown between two genotypes.
- Gametes –Must be circled.
- The fusion process (fertilization).
- The filial generation types.
An alternative method of making genetic crosses is a Punnet square
When the allelic genes are identical as in TT and tt, the condition is known as homozygous. An individual with such condition is referred to as a homozygote.
When the allelic genes are not identical as in Tt, the condition is known as heterozygous. An individual with such condition is referred to as a heterozygote.
An individual with genotype Tt will be phenotypically tall because the gene T is dominant over the gene t. The allele t is recessive.
A dominant gene is that which can express itself in both its homozygous (TT) and heterozygous (Tt) state, while a recessive gene is that which can only express itself in its homozygous (tt) state. Therefore, the genotypic condition TT is referred to as homozygous dominant while tt is homozygous recessive.
The ratio 3 tall: 1 dwarf in the F2 generation is characteristic of monohybrid inheritance where one gene is completely dominant over the other. This is complete dominance. The monohybrid crosses are based on Mendel’s 1st law, which states that, the characteristics of an organism are determined by internal factors (genes) which occur in pairs. Only one of a pair of such factors can be represented in a single gamete.
Ratios and probability
The 3:1 ratio in monohybrid inheritance can be presented in the form of probability i.e.
¾ or 75% of the offspring are tall while ¼ or 25% are dwarf.
The inheritance of characteristics involves probability. The chance that a particular gamete will fuse with another is a random occurrence. In genetic crosses, this is done by showing all possible fusions.
Similar monohybrid results as those of Mendel have been obtained by using a common insect, the fruit fly (Drosophila melanogaster).
Reasons why Drosophila melanogaster (Fruit fly) was suitable for genetic studies
- Lays many eggs resulting in large number of offspring.
- Has many observable distinct and contrasting traits.
- Has a short life cycle (generation time).
- Safe to handle because they do not transmit diseases.
- Offspring can be crossed with their parents at will (backcrossing).
- Easily bred in the laboratory with minimum requirements.
Reasons why humans (Homo sapiens), unlike garden peas (Pisum sativum), are not convenient subject for genetic studies
- The human generation is too long unlike that of peas which is short.
- Human produce fewer offspring as compared to pea plants.
- Breeding experiments morally unacceptable with human.
- Long life cycle;
- Fewer contrasting traits
